1. Field of the Invention
The present invention relates to a thermal fixing device and an image forming apparatus including the thermal fixing device.
2. Description of the Related Art
In order to thermally fix a toner image transferred to a sheet, an image forming apparatus, such as a laser printer, is usually equipped with a thermal fixing device having a thermal roller and a pressure roller. The toner image transferred to the sheet is thermally fixed while the sheet passes between the thermal roller and the pressure roller. The thermal fixing device also includes a temperature controller that controls temperature along the entire axial length of the thermal roller to within a fixed temperature range.
The following problem arises when the same thermal fixing device is used to thermally fix toner images on two differently sized sheets, that is, both a small size sheet (e.g., A6) and a large size sheet (e.g., A4). Assume that first the thermal fixing device is used to fix toner images on a series of small size sheets. When the sheets contact the thermal roller, the sheets draw heat away from the thermal roller surface that contacts the small size sheets. To insure that temperature of the thermal roller does not drop below the fixed temperature range, the temperature controller controls to heat up the thermal roller at portions in contact with the small size sheet to within the fixed temperature range. Because the temperature controller controls heat across the entire length of the thermal roller, the temperature at non-contacting portions of the thermal roller, that is, the temperature at the two axial end portions of the thermal roller, will increase to higher than the fixed temperature range.
Next, assumed that the thermal fixing device is used to fix toner images on a series of large size sheets. Because the end portions of the thermal roller are excessively hot when they contact the large size sheet, the toner is excessively melted by the hot end portions. The excessively melted toner can stick to the surface of the thermal roller and be transferred onto sheets that are subsequently printed. This is referred to as hot offset.